Fix item temperature processing in ice labs

[Hirmuolio]

Summary

SUMMARY: Bugfixes "Fix temperature processing in ice labs"

Purpose of change

Fixes #39778

Describe the solution

Rottable items on floor are processed on map load by map::remove_rotten_items. This processing used wrong coordinate. The temperature code didn't like that coordinate and would just always return 0 as the temperature modifier.

Describe alternatives you've considered

Testing

Skipping time in ice lab, coming from far to ice lab after skipping time and going down stairs to ice lab after skipping time work with z-levels on and off.

Additional context

There is still a smaller bug: When z-levels are off g->m.get_temperature( pos ) gives the temperature of wrong z-level for items processed by map::remove_rotten_items if the player changes z-level on that turn (with stairs, does not happen with debug teleport).

So when you go from above ground (level 0, temperature modifier -20) to underground (level -1, temperature modifier -50) the items on the below ground floor get their temperatures processed with temperature modifier -20 instead of -50.